This invention relates to data transmission systems, and in particular to a technique for precoding, prior to differential coding of the transmitted data, and subsequent decoding following differential decoding in order to at least partially change double errors, which occur as a result of the differential encoding and decoding of a single error, to a quantity of errors having an odd number.
Differential coding is used in digital carrier systems because it permits the simplification of the regeneration at a receiver. This is because the phase ambiguities, that may arise due to erroneous polarity changes that may occur in transmission, are resolved in the differential decoding process. Differential coding will guarantee that the received decoded data is identical to the original data at the transmitter input, whether or not the received data has been inverted during the demodulation process. Differential coding can be visualized by realizing that at the transmitter a 1-polarity signal element is tranmmitted as a signal element which changes polarity while a 0-polarity is the signal element that does not cause any change of polarity. At the receiver a 1-polarity is generated if two successive signal elements have different polarities, while the reception of two signal elements with the same polarity causes a 0-polarity signal element to be generated. It may be readily seen that such a system will change each single error that occurs during transmission to a double error in the differential decoding process. This is important to error detection using parity checks, because one of the more commonly used error detection techniques employs the addition of an extra bit to the binary word according to the rule that the total number of 1's, after the addition, must be either odd or even. Because single errors introduced in differentially encoded digital data are converted to double errors, they will pass through the system undetected.
One technique by which the double errors introduced by differential coding are reduced to single errors is described in U.S. Pat. No. 3,436,730, entitled "Method of Detecting and Correcting An Error in Polarity Change in a Data Transmission System", granted Apr. 1, 1969 to W. H. E. Widel. The detecting and correcting means in the invention there includes integrating means for separately integrating the positive and negative lobes of each received data element to produce two signals which are combined to produce a difference signal. The magnitude of the difference signal is compared with a predetermined threshold value. If the magnitude is less than the threshold value a signal is generated which activates means for reversing the sense of the related data signal element which thus reduces the double error to a single error. While the apparatus used to perform the change of the double error to the single error is located at the receiver, it should be noted that the device is complex in that considerable logic is required and there is a delay in the operation of the device.